Dendritic spines are tiny appendages on the surface of many neurons that connect excitatory afferents with dendrites. They form separate biochemical micro-compartments and are considered sites of synaptic plasticity. Many spines of forebrain neurons contains a spine apparatus, an enigmatic organelle that consists of stacks of smooth endoplasmic reticulum intervened by actin filaments containing electron-dense plates. The precise function of the spine apparatus is unknown. Synaptopodin, which was identified and characterized by the investigator, is a proline-rich actin-binding protein of telencephalic dendrites and renal podocytes. The close association with the spine apparatus suggested that synaptopodin is an important component of this organelle. Here we will explore at the molecular level how synaptopodin is mechanistically involved in the biogenesis and dynamics of the spine apparatus. Based on our Preliminary Data we hypothesize that synaptopodin is required for the formation or maintenance of the spine apparatus. Synaptopodin may affect spine apparatus morphology by bundling actin filaments in the spine neck. To test this hypothesis, we propose the following two Specific Aims: the first Specific Aim will explore the role of synaptopodin in the formation and plasticity of the spine apparatus in mice. In the second Specific Aim we will elucidate the cellular mechanism by which synaptopodin contributes to the formation and plasticity of the spine apparatus The combination of biochemical, cellular, and molecular genetic approaches proposed in this application will provide insight into the biogenesis of spine apparatus formation and its regulation by synaptopodin. If our hypotheses are correct the work proposed here will have broad significance because it will a) define a molecule link between synaptopodin function and the biogenesis and plasticity of the spine apparatus and b) establish insight into the role of the spine apparatus in synaptic plasticity. This should in the long-term enable us to develop novel therapies that tackle learning deficiencies and memory loss by modulating the expression of synaptopodin thereby promoting the cellular plasticity of the dendritic spine apparatus.